This invention relates to a method for measuring the alcohol concentration of an acetic acid fermenting broth.
Generally, the biological activity of acetic acid bacteria is not always constant in a submerged acetic acid fermentation. Therefore, if an alcohol solution as a substrate is added to large excess, a decrease of utilization efficiency of the raw material or a decrease in productivity results due to the high alcohol accumulation in the fermenting broth. On the other hand, if insufficient alcohol is added, the same decrease of productivity occurs due to the deficiency of alcohol in the fermenting broth. In addition to this, if in surface acetic acid fermentation, alcohol which is a substrate, is not still present at the point of discharge, the acetic acid is peroxidized by acetic acid bacteria and it changes the quality of the vinegar greatly for the worse.
As noted hereinbefore, in acetic acid fermentation, the concentration of the alcohol, which is a substrate, is an important factor which significantly influences fermentation conditions. Accordingly, a change of the alcohol feed rate or a determination of discharge is based upon repeated measurement of the alcohol concentration of a fermenting broth taken from the fermentor daily by means of colorimetric determination or gas chromatography in order to maintain good fermentation conditions.
It has been difficult to put these conventional methods for alcohol measurement into practical operation, because it takes too much time until measurement data is obtained after taking a sample from the fermenting broth. In addition, in these methods it has been impossible to determine the concentration from a fermentation stage which varies as the processing proceeds and consequently it impedes good fermentation or results in the production of lower grade or bad vinegar.
A method using a gas sensor such as a Flame Ionization Detector (hereafter FID) or a semiconductor gas sensor has been developed as a method for the measurement of concentration of volatile components in liquid. The principle of this method is based on measuring the concentration of volatile components in gas by introducing a gas attaining equilibrium with the vapor into a gas sensor. In this method, the real concentration of volatile components in a fermenting liquid may be determined by conversion of the concentration of volatile components in the gas to that in liquid.
The following two reports disclose methods using a gas sensor: Ethanol Analysis In Baker's Yeast Fermentation. Biotechnology and Bioengineering, Vol. XXIII, p 2509-2524 (1981); and Methanol Measurement in Biomass Production By Fermentation Using Methanol As A Substrate, J. Ferment. Technol., Vol. 56, No. 4, p 421-427 (1978).
The gas sensor method seems to be very useful for fermentation wherein the volatile component consists of only a single volatile component as mentioned above. However, when a fermenting broth contains plural volatile components which may be detected by the gas sensor, it is substantially impossible to measure only a specific volatile component of the mixture because the gas sensor is not selective.
Fermenting broth in acetic acid fermentation contains two volatile components, alcohol and acetic acid. The FID or semi-conductor gas sensor may detect both and then the electrical signal from the gas sensor may be the output sum of both components. Because of this difficulty when attempting to measure only alcohol which is an important parameter in acetic acid fermentation, it has not been useful to utilize a method that directly analyzes gas concentration using such gas sensors for the measurement of the alcohol concentration of acetic acid fermenting broth. In the following description, without exception, the concentration of alcohol is expressed in volume by volume and the concentration of acetic acid in weight by volume.
It is an object of this invention to provide a method for the measurement of alcohol concentration in acetic acid fermentation.